The invention relates to a digital information transmission system using differential pulse code modulation and comprising an encoder for converting an incoming linear PCM signal into a differential PCM signal with a reduced number of bits, and a remote decoder for carrying out the inverse conversion, the encoder comprising a difference circuit for forming a difference signal between the incoming linear PCM signal and a prediction signal, a quantizer for quantizing the difference signal to form the differential PCM signal, and a local decoder for receiving the differential PCM signal and forming the prediction signal, the local decoder and the remote decoder being constituted in the same manner and each comprising a quantizer, operating inversely to the quantizer of the encoder, and a filter constituted by a cascade arrangement of filtering sections, each section being provided with a round-off (or truncation) circuit operating with a step equal to the quantizing step of the differential PCM signal.
Such a transmission system using differential PCM is described in the French Patent Specification No. 2,441,971 of the Applicants.
It is known that the transmission of signals coded according to the differential PCM method enables a lower bit rate in a transmission path than in the event of these signals being coded according to the PCM method. If, for example, a standardized PCM signal of 8 bits is converted into a differential PCM signal of 4 bits, each being sampled at the same frequency of 8 kHz, the transmission according to the differential PCM method can be effected at a rate of 32 kbit/s instead of at the rate of 64 kbit/s required for the transmission according to the PCM method. However, modern digital telephone communication networks are designed for the transmission and the switching of standardized PCM signals of 8 bits. If the reduction of the bit rate offered by the transmission according to the differential PCM method should be utilized in the existing networks, a conversion of a PCM signal into a differential PCM signal has to be effected at the input of each transmission path, while the inverse conversion of a differential PCM signal into a PCM signal has to be effected at the output of each transmission path connected to a switching center. Thus, a telephone connection in a network constituted in this manner may render it necessary to carry out a cascade of conversions of PCM to differential PCM and inversely.
In the transmission system using differential PCM described in the French Patent Specification No. 2,441,971, the accumulation of noise produced at each conversion and the possible misalignment of the digital filters of the local decoder, and the remote decoder are avoided in that these filters are constituted by a cascade of filtering sections each having a round-off (or truncation) circuit operating with a step equal to the quantizing step of the difference signal formed in the encoder. Preferably, this quantizing step is variable and automatically adapted to the level of the difference signal in order that the latter is coded as best as possible with the reduced number of bits of the differential PCM signal.
However, it does not alter the fact that in the differential PCM transmission system considered, the conversion of the incoming PCM signal into a differential PCM signal produces a quantizing noise that has to be reduced as much as possible to improve the quality of the transmission. Since this quantizing noise is proportional to the difference signal which is quantized in the encoder, attempts must be made to reduce the difference signal as much as possible so as to obtain a prediction signal which approximates to the incoming PCM signal of the encoder as close as possible. It can be shown that the ideal prediction filter has a response which is the image of the spectrum of the signal to be transmitted. Now the signals to be transmitted may be of different nature: data or speech signals, and moreover a distinction has to be made, especially for the speech signals, between the long-term spectrum and the short-term spectrum for syllabic durations. In the French Patent Specification No. 2,441,971 it is recommended to use a fixed prediction filter constituted by a cascade of filtering sections of arbitrary order having, however, only a sole coefficient and providing a compromise response between these different spectra. However, such a fixed prediction filter is rarely welladapted to the signal at a given instant and does not always permit of obtaining the required transmission quality.